The present invention relates generally to television signal transmission systems and particularly concerns digital television signal transmission systems employing fully or partially suppressed carriers.
U.S. Pat. Nos. 5,086,340 and 5,087,975 disclose a television signal transmission system for transmitting and receiving a digital HDTV video signal over a standard 6 MH.sub.z television channel without excessive interference from an NTSC co-channel signal. The video signal is processed in the transmitter using a modulo-N precoding filter to facilitate the use of a complementary postcoding linear filter in the receiver to substantially cancel interfering NTSC co-channel beats without significantly affecting the received HDTV signal. Alternatively, in the absence of NTSC co-channel interference, the received signal may be processed by a complementary modulo-N postcoding filter which has better signal-to-noise performance than the linear postcoding filter.
In order to minimize the possibility of the HDTV transmission from interfering into an NTSC co-channel, it is desirable to transmit the HDTV signal using a suppressed carrier format or, alternatively, in conjunction with a relatively small pilot component. In either event, accommodations must be made to allow for proper reception of the transmitted signal. In particular, satisfactory receiver lock-up may be substantially enhanced by insuring that the average carrier level of the transmitted signal is characterized by at least a selected predetermined value. The digital video data itself is characterized by generally randomly occurring levels both above and below the zero carrier level and thus would normally provide a substantially zero average carrier level at the receiver. The system of the application referred to above included a quadrature pilot component at the carrier frequency to overcome this deficiency. However, for various reasons, it may be desirable to instead use an in-phase pilot component or, for that matter, no pilot component at all. In either of these cases, receiver lock-up may become a problem; in the former case because digital video data below the zero carrier level may cancel the pilot and in the latter case because of the randomly occurring nature of the digital video data above and below the zero carrier level.
It is therefore a basic object of the present invention to provide an improved digital television signal transmission system.
It is a more specific object of the invention to provide a digital television signal transmission system employing a relatively small in-phase pilot component or no pilot component at all.
It is another object of the invention to provide a digital television signal transmission system employing a relatively small in-phase pilot component or no pilot component at all and in which a digital video signal is coded in the transmitter by a modulo-N precoding filter and the received signal is processed by either a complementary linear postcoding filter or by a complementary modulo-N postcoding filter.
It is yet a further object of the invention to provide a television signal transmission system of the foregoing type wherein receiver performance is not substantially degraded by transmission errors.